The present invention relates to a method for varying colors of a picture image, displayed in a color display such as an electronic color proof means in which a picture image having the similar colors to those of a printed matter finished is displayed in a color cathod ray tube (CRT) before printing in order to check or anticipate color separation conditions when color separation plates are produced, a color separation condition setup monitor for a color scanner, a color correction monitor for a color layout scanner, or the like, in a digital manner.
In general, in a multicolor printing three color inks of subtractive primary colors such as cyan, magenta and yellow and a black color ink for supplementing mainly a dark tone are used, and such inks are printed at the desired ratio, thereby obtaining a variety of colors. Therefore, the reproduced color tone of the printed matter finished almost depends on the color separation plates which directly influence the amounts of the inks.
In a conventional color proofing of a multicolor printed matter, printing plates are made from color separation plates or films, and then a proof is printed by the printing plates by using the inks. Then, the obtained proof is checked. When it is judged that the obtained proof is improper or it is empirically judged from the states of an original color picture and the color separation plates that it is necessary to correct the proof, the color separation plates are manually and directly retouched or are re-produced under other corrected color tone and color separation conditions. Then, by using the retouched or the re-produced color separation plates a fresh proof is obtained and checked again, in the manner of trial and error.
Accordingly, in this method, every time, the color separation plates are corrected or re-produced, and then printing plates for proofing are prepared depending on the necessary colors. Then the proof is obtained by using a proof press or a printing press. Therefore, this method involves a lot of time and trouble and much cost.
In order to remove such defects, the electronic color proof means has been realized. In this case, the color separation plates are recorded by recording video signals obtained by photographing consecutively the color separation plates by using a TV camera, and then the video signals for the color separation plates are periodically reproduced in the same time into the color separation plates while the colors of the printed matter to be reproduced are calculated in an electronic circuit and the calculated results are displayed in a color cathod ray tube, hereinafter referred to as a color CRT.
Further, a method in which a plurality of color separation plates are synchronously scanned by a flying spot tube and are displayed as color images without recording, has been developed. In this case, the color CRT is actuated by additive primary color signals R, G and B of red, green and blue, and hence subtractive primary color signals C, M, Y and K of cyan, magenta, yellow and black, which are output from the electronic circuit of the electronic color proof means, must be converted into the additive primary color signals R, G and B.
One such a signal conversion means has been known, as disclosed in Japanese Patent Publication No. 51-4777, wherein in order to correct the additivity law failure which is arisen at a position where a plurality of color inks are printed one above the other, the so-called Neugebauer's equation is utilized. Another signal conversion method has been proposed, as disclosed in Japanese Patent Publications Nos. 54-38921 and 54-38922. In this case, while the under color removal is jointly used in order to prevent the gradation from being gone or compressed from the intermediate portion to the shadow portion, the subtractive color signals C, M, Y and K are converted into the additive color signals R, G and B.
Another signal conversion method has been also developed, as disclosed in Japanese Patent Publication No. 56-26015 (Japanese Patent Application No. 51-123795). In this case, such a signal conversion is carried out in an analog circuit, supplementing a calculation for correcting the additivity law failure in consideration of the unnecessary absorbing component of the ink, that is, the so-called "impurity of ink". However, these conventional methods described above are performed in the analog manner.
Recently, with the advance of the digitalization of the electric circuits, the electronic color proof means of such signal conversion means for the above purposes have been digitalized. However, in fact, this digitalization is carried out by simply converting the analog circuits into the digital circuits. Hence, when such a signal conversion is performed in a digital circuit which is composed of a multiplier, or the like. Therefore, the processing of the signal conversion requires a lot of time. Accordingly, when the high speed processing is necessary for this signal conversion, many inconveniences arise.
Furthermore, in the conventional electronic color proof means, when the colors of the printed matter reproduced are displayed, the problem of the additivity law failure occurs.
The additivity law failure is caused at the position where a plurality of color inks are printed one above another, and its strength varies complicatedly depending on the amounts of the inks, the printing order, and so forth.
There is still another problem, that is, as shown in FIG. 1, the densities of red, green and blue components of the ink do not vary linearly with respect to the halftone dot area rates.